I claim the ornamental design for a lighting fixture, as shown and described.
The single FIGURE is a front/right/bottom perspective view of a lighting fixture. This fixture has five-way (72°) overall rotational symmetry and the shades have a circular horizontal cross section.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design, while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.